Abstract: A main structure is intended to engage and disengage an flow-injection type piezoelectric sensor and is composed of a movable stand, a lifting mechanism, and a fixed injection portion. The movable stand is capable of being actuated by a rotary shaft of the lifting mechanism to move upwards or downwards. The movable stand is provided with a retaining slot capable of catching and releasing the flow-injection type piezoelectric sensor. When the movable stand is lifted, a transparent end of the fixed injection portion is in an intimate contact with a planar surface of the flow-injection type piezoelectric sensor which is caught in the retaining slot of the movable stand. A liquid test sample is thus injected into a charge channel of the transparent end such that the liquid test sample flows on the surface of a piezoelectric crystal located in an opening of the planar surface of the flow-injection type piezoelectric sensor for detecting the presence of a target molecule in the test sample.